Valve timing mechanisms having switchable valve timing mechanism elements, for example switchable tappets, levers or supporting elements and correspondingly switchable cylinders, are installed in motor vehicles in order to optimize the efficiency, the fuel consumption and the exhaust gas emissions, in order, depending on the application, to switch on or switch off valves in defined operating states, to load them with different valve strokes or to actuate them with different valve control times. Switchable valve timing mechanism elements of this type are affected with a locking play of the locking means which are used, for structural reasons, in order to implement a locking action.
As is described, for example, in DE 102 10 747 A1, two pistons which lie diametrically opposite one another can be configured here as locking means. In the event of a locking action, the pistons bridge two axially movable components, which are arranged inside one another, of the valve timing mechanism element which is configured, for example, as a roller tappet. This results in an additional idle travel which acts on a valve stroke and has to be taken into consideration in the design of the cams on a camshaft, via which the gas exchange valves of the internal combustion engine are actuated.
This is expressed in the design of a ramp section within an opening ramp which adjoins a base circle region of the respective cam, which ramp section additionally has to be overcome before the actual opening process of the gas exchange valve when the cam is moved out via the valve timing mechanism element. Said ramp section therefore increases the overall length of the cam ramp. In addition to the compensation of the locking play, an increase in the locking play as a result of wear is also to be taken into consideration. In general, the mechanical rigidity of the valve timing mechanism elements is also to be considered in the compensation of the idle travel.
Furthermore, both switchable and nonswitchable valve timing mechanism elements are frequently installed in internal combustion engines, with the result that the respective ramp lengths are different. If further ramp portions are added to the ramps which are already different in any case, the cam ramps of the corresponding cams become relatively long and it becomes more difficult with an increasing length of the cam ramps to match an internal pressure of the switchable and nonswitchable cylinders via the cams.
A further ramp portion is required, in particular, in order to compensate for lowering of the frequently used hydraulic valve play compensating elements (HVA) in the internal combustion engine. HVAs serve to compensate automatically for a valve play which results from the thermal expansion and the wear of the transmission elements of the cam stroke, which emanates from the camshaft, on the gas exchange valves, in order to ensure a satisfactory operation of the internal combustion engine. The valve play compensating elements are usually configured as hydraulic tappets, having a control valve which is loaded in the closing direction via a spring and is therefore predominantly closed in the base circle region of the cam by the force of the spring. As a result, said compensating elements transmit a cam stroke directly to the gas exchange valves, as virtually rigid elements.
Valve play compensating elements are also increasingly used, in which the control valve is loaded in the opening direction. Compensating elements of this type are known as RSHVA (reverse spring hydraulic valve play compensating elements) and are described, for example, in DE 10 2004 018 386 A1, DE 10 2004 035 588 A1 and DE 10 2005 010 711 A1 which are not prior publications. In these compensating elements, the control valve is held open in the cam base circle region by the force of the spring. As a compensating element of this type can be closed by hydrodynamic and hydrostatic forces only by an oil flow which flows at the beginning of the cam elevation from a high pressure space to a low pressure space of the tappet, the compensating element initially produces an idle stroke, before the actual valve stroke begins. This idle stroke has the effect of reducing the wear on the valve timing mechanism and, in particular, has a positive effect on the idling quality (rotational speed stability during idling) of the engine. In order to compensate for the idle stroke of the RSHVAs, however, a further ramp portion is necessary, which results in a still further increased overall ramp length of the relevant cams.
As a result of the individual ramp portions, in particular in the case of a valve timing mechanism having a switchable tappet with RSHVA, a critical overall length of the cam ramp can therefore be achieved, which can result in creeping opening of the corresponding gas exchange valves or a filling loss of the corresponding cylinders. This can have an unfavorable influence on the valve overlap and therefore the valve control times. In addition, unfavorable thermal effects as a result of the creeping filling outflow are also not to be precluded. As a consequence of this, the actual advantages of switchable valve timing mechanism elements and RSHVAs, in particular with regard to the idling quality, can be reduced unfavorably, at least partially.
DE 199 02 446 A1, DE 196 30 443 A1 and DE 196 29 313 B4 are to be mentioned for a cam design having an opening ramp and a closing ramp. In said documents, the cam design is described for taking a hydraulic play compensating element into consideration. However, a person skilled in the art does not find any indications in said publications of a locking play or an additional reverse spring idle stroke being taken into consideration in the design of the opening ramp.